Conventional one-way paging systems are well known in the art. Radio paging technology generally consists of simple tone pagers with a tone alert, a voice pagers with a voice message, numeric messages or an alphanumeric display pagers. In a typical paging system such as that shown in FIG. 1, a caller 10 places a call to a paging service provider at a paging terminal 12. The paging terminal queues up the messages to be sent to a number of remote paging units 14 via paging transmitter 13 which transmits a strong signal to the service area where all the remote pagers reside with enough power to reasonably reach all pagers within that market. Once the message is sent, there is no confirmation that it was ever received. The remote paging unit may be turned off, interference may have prevented reception, bad propagation conditions such as multipath, the unit may be out of the geographic service area, or any number of reasons may have prevented reception.
The messages transmitted from paging transmitter 13 to remote pager 14 generally use the POCSAG standard code for paging terminals. The POCSAG code includes a preamble, an address and a message for remote pager 14. The pagers within the geographic market area all receive the POCSAG codes on an assigned radio frequency and scan each message for the address assigned to the individual units. When the address (called a CAP code) is received, then the other information transmitted along with the code is recorded and displayed and the user is alerted.
In many markets, the single paging transmitter 13 is incapable of covering the entire geographic area of the local market. This may be due to the requirement of paging signal reception inside heavily constructed buildings, the fact that the antenna of a pocket receiver is inefficient and the proximity of the human body may affect field patterns for reception. In such cases, it is typical to find a paging communication system such as the system shown in FIG. 2. In this configuration, the caller calls through a PBX 15 through to the paging terminal 16 where the message is received and queued for transmission through VSAT 18 and uplink 19 to a geostationary satellite 20 for simultaneous retransmission (multicast) to a plurality of paging transmitter towers 13a,13b and 13c. In this fashion, an entire geographic market can be serviced with the simultaneous transmission of the same paging messages over all transmission towers 13a-13c. The use of a geostationary satellite of course is not the only method of simultaneous transmission to the plurality of paging towers. Terrestrial links may also be employed.
A block diagram of a typical pocket paging unit such as the type used with the configurations of FIGS. 1 and 2 is shown in FIG. 3. This configuration is well known to those skilled in the art and includes a receiving antenna 21 driving a crystal controlled receiver 22 which down converts the received signal and strips out the digital data using an identification detector 24 to locate the CAP code address in the POCSAG data. If the address matches the receiver's address, an audible alert may be sounded through speaker 29. The microcontroller 26 is available in alphanumeric pagers to display the information received on LCD display 27. Associated with controlling the operation of the pager are one or more push buttons 28. The power unit is included which essentially powers the entire pager with very small NiCd or alkaline batteries.
An essential feature of these types of pocket remote paging units is the ability to operate the receiving circuits at a very low power so that the user need not change the NiCd or alkaline batteries on a too-frequent basis. One of the essential marketing advantages of the use of pocket pagers is the low maintenance and reliability such that the user need not worry about frequent changing of the batteries.
A major drawback in the use of the paging system of FIG. 1 is the inability of the caller 10 to have any confidence that the holder of remote pocket pager 14 has received the message and is responding. In such critical industries such as the medical field, or for maintenance and repair personnel, it is essential that the caller know that the called party has received the message and is responding in a timely fashion. Unfortunately with the systems of FIG. 1, there is no such acknowledgement to give the caller any confidence.
To address this problem, a variety of acknowledgement paging systems have been proposed whereby the paging unit 14 has a means of acknowledging the receipt of the message and even informing the user of a menu of possible responses to the caller's query. Typical of these types of systems is the system described in U.S. Pat. No. 4,644,341 to Zabarsky et al. entitled "Two Way Personal Message System With Extended Coverage" which describes a relatively high power, wideband personal messaging system. This system is typical of early messaging systems which were inefficient both in power usage and bandwidth thereby restricting their use to a relatively narrow market. A later attempt to extend personal two way messaging to voice pagers is taught in U.S. Pat. No. 4,811,379 to Grandfield entitled "Speak Back Paging System".
To alleviate the bandwidth and power limitations, more sophisticated two-way messaging systems have been proposed. Typical of these types of systems is the system described in U.S. Pat. No. 4,825,193 to Siwiak et al. entitled "Acknowledge Back Pager with Adaptive Variable Transmitter Output Power". In the Siwiak patent, an acknowledgement paging system is described in which the remote paging unit has the ability to send one of a selected number of messages from a menu back to the paging terminal to acknowledge the receipt of the message. This system is further described in U.S. Pat. Nos. 4,823,123, 4,875,038 and 4,891,637 also to Siwiak et al. See also U.S. Pat. No. 5,142,279.
The system of the Siwiak patents transmits the acknowledgement signal on one of a list of frequency sub-bands approximately centered around 150 megahertz. The transmitting paging terminal sends the message to the acknowledgement pager of the Siwiak system and also tells it which frequency of the list of sub-band frequencies upon which the acknowledgement pagers should respond. In this fashion, the paging terminal reserves one of the sub-band frequencies for that particular acknowledgement message. By allocating and re-allocating the sub-band frequencies, collisions between simultaneous-transmitting multiple remote paging units is minimized.
There is a need in the art for the ability to conserve power in reverse pagers and the Siwiak system attempts to ameliorate this problem with limited success. The users of acknowledgement pagers demand the same performance and reliability of the acknowledgement pocket pagers as they do from the one-way pocket paging units. The problem with acknowledgement pagers, however, is the ability to transmit enough power from the pager so that the acknowledgement signal is received at the paging base terminal site. As was previously mentioned, the transmitting power from the paging base transmitter 13 is sufficiently strong to reach most if not all of the geographic market. If enough transmit power is unavailable to reach the entire geographic market, a plurality of transmit towers are used such as that shown in FIG. 2. Due to the large geographic markets typically found for paging systems, it is difficult to transmit enough power from a handheld or pocket-sized acknowledgement paging unit for that acknowledgement message to be received at the paging terminal. Thus, to generate enough power, either larger batteries are needed or more frequent changing of the batteries will result. The Siwiak patent includes a variable transmit power option which conserves battery power by transmitting allow power signal when the signal strength of the received signal from the transmit tower has a high signal strength. This would indicate that the transmit tower, and its associated receiving tower to receive the acknowledgement message, is reasonably close. If, however, the power level of the transmit signal from the paging transmitter 13 received by the acknowledgement pager is weak, that would indicate that a higher power transmit signal from the acknowledgement pager of the Siwiak invention would be needed and the transmit power is boosted. This of course shortens the battery life of the system. Even with a minimal amount of acknowledgements sent back to the transmitting site, the batteries do not last as long in acknowledgement pagers as in the traditional pagers.
Another concern in the area of acknowledgement paging is the ability to asynchronously transmit acknowledgment signals. The Siwiak patents described above suffer from the need to be tightly coupled to the base station in terms of assigned frequency use and time of transmissions of the acknowledgment signals to avoid message collisions between simultaneously transmitting pagers. An attempt at pseudo-asynchronous behavior is described in U.S. Pat. No. 4,868,560 to Oliwa et al. entitled "Voice Paging System Providing Deferred Paging Capability".
Another concern in the area of acknowledgement paging is the cost of the pager itself. The ability to conserve power so that batteries do not have to be replaced as often causes great constraints on the components used for the construction of such pagers. Once again, the market expects a certain level of cost for a pocket paging unit and acclimating that market to higher cost pagers will cause market resistance to the penetration of acknowledgement pagers. Thus, a concomitant goal would be to provide an acknowledgment pocket pager at approximately the same cost as the one-way pocket pager while having the ability to maintain approximately the same battery life.
Another problem with market acceptance of acknowledgement pagers is the ability to adapt an acknowledgement paging system to the existing infrastructure of a one-way paging network. As shown above, the ability to transmit enough power from a pocket pager to be received at the sites of the existing paging transmitter towers is directly at odds with the ability to conserve battery power so that the batteries do not have to be changed as often. This tradeoff has not been met by the paging schemes proposed to date and thus market acceptance of acknowledgement paging is inhibited.
Also, due to the limited bandwidth for transmitting acknowledgement signals and the need for a large number of two-way pagers in a single market, there is a need in the art for the ability to simultaneously transmit from pagers and receive the acknowledgment signals without interference from simultaneous transmissions. An attempt to address such a need is found in U.S. Pat. No. 4,882,579 to Siwiak entitled "Code Division Multiplexed Acknowledge Back Paging System". The system described in this patent attempts to address this problem by using direct sequence spread spectrum data transmission within single frequencies. This system is still quite limited in the number of non-colliding acknowledgment messages it can handle, it is rigid in the time and frequency of acknowledgment transmissions (entirely synchronous transmit and receive protocol) and is inefficient in its bandwidth usage.
There is also a need in the art for automatic handling of outgoing messages and return receipt of the acknowledgement messages. U.S. Pat. No. 5,153,582 to Jasinski et al. entitled "Method of and Apparatus for Acknowledging and Answering A Paging Signal" attempts to provide a semi-automatic means but a human operator is still required to receive the paging information.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a low-power, asynchronous reverse paging system adaptable to acknowledgement paging or polled response handling without collisions of simultaneous messages.